This invention relates to color cathode ray picture tubes and is addressed specifically to a novel front assembly for color tubes that have a tension foil shadow mask. The invention is useful in color tubes of various types including those used in home entertainment television receivers, and those used in medium-resolution and high-resolution tubes intended for color monitors.
The use of the tension foil mask and flat faceplate provides many advantages and benefits in comparison with the conventional domed shadow mask. Chief among these is a greater power-handling capability which makes possible as much as a three-fold increase in brightness. The conventional curved shadow mask, which is not under tension, tends to "dome" in high-brightness picture areas where the intensity of electron bombardment is greatest. Color impurities result as the mask moves closer to the faceplate. Being under high tension, the tension foil mask does not dome or otherwise move in relation to the faceplate, hence its greater brightness potential while maintaining color purity.
The tension foil shadow mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises the faceplate with its deposits of light-emitting phosphors, a shadow mask, and support means for the mask. As used herein, the term "shadow mask" means an apertured metallic foil which may have a thickness, by way of example, of about one mil or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q-distance". The high tension may be in the range of 20 to 40 kpsi. As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, which ensures that each of the three beams lands only on its assigned phosphor deposits.
The requirements for the support means for the shadow mask are stringent. As has been noted, the shadow mask must be mounted under high tension. The mask support means must be of high strength so that the mask is held immovable--an inward movement of the mask of as little as one-tenth of a mil is significant in that guard band may be expended. Also, the shadow mask support means must be of such configuration and material composition as to be compatible with the means to which it is attached. As an example, if the support means is attached to glass such as the inner surface of the faceplate, the support means must have about the same thermal coefficient of expansion as that of the glass. The support means must provide a suitable surface for mounting the mask. Also, the support means must be of a composition such that the mask can be welded onto it by electrical resistance welding or by laser welding. The support surface is preferably of such flatness that no voids can exist between the metal of the mask and the support structure to prevent the intimate metal-to-metal contact required for proper welding.
A tension mask registration and supporting system is disclosed by Strauss in U.S. Pat. No. 4,547,696 of common ownership herewith. A frame dimensioned to enclose the screen comprises first and second space-apart surfaces. A tensioned foil shadow mask has a peripheral portion bonded to a second surface of the frame. The frame is registered with the faceplate by ball-and-groove indexing means. The shadow mask is sandwiched between the frame and a stabilizing or stiffening member. When the system is assembled, the frame is located between the sealing lands of the faceplate and a funnel, with the stiffening member projecting from the frame into the funnel. While the system is feasible and provides an effective means for holding a mask under high tension and rigidly planoparallel with a flat faceplate, weight is added to the cathode ray tube, and additional process steps are required in manufacture.
There exists in the marketplace today a color tube that utilizes a tensed shadow mask. The mask is understood to be placed under high tension by purely mechanical means. Specifically, a very heavy mask support frame is compressed prior to and during affixation of the mask to it. Upon release of the frame, restorative forces in the frame cause the mask to be placed under high residual tension. During normal tube operation, electron beam bombardment causes the mask to heat up and the mask tension to be reduced. An upper limit is placed on the intensity of the electron beams that may be used to bombard the screen without causing the mask to relax completely and lose its color selection capability. The upper limit has been found to be below that required to produce color pictures of the same brightness as are produced in tubes having non-tensed shadow masks. For descriptions of examples of this type of tube, see U.S. Pat. No. 3,683,063 to Tachikawa.
A color cathode ray tube includes three electron guns arranged in a delta- or an in-line configuration. Each gun project an electron beam through the apertures of a mask onto assigned target areas located on the inner surface of the faceplate. The target areas comprises a pattern of phosphor deposits typically arranged in triads of dots or lines. Each of the triads consists of a deposit of a red-light-emitting, green-light-emitting, and a blue-light-emitting phosphor. To increase the apparent brightness of the display, and to minimize the incidence of color impurities that can result if a beam falls upon an unassigned phosphor deposit, the target area may include a layer of darkish light-absorbing material termed a "grille" that surrounds and separates each of the dots or lines, and which serves as a "guard band" in case of beam misregistration.
The phosphor deposits are typically formed by a photoprinting process. The grille, which is also termed the "black surround," is applied first. The target area is then coated with a photosensitive slurry comprising phosphor particles of one of the three phosphors described. The shadow mask, mounted on a rigid frame, is temporarily installed in precise relationship to the faceplate, and the coating is exposed to light actinic to the phosphor deposits projected through the apertures of the mask from a light source located at a position that corresponds to the beam-emission point of the associated electron gun of the end-product tube. The faceplate is then separated from the shadow mask and the coating is "developed." The final result is a pattern of dots or lines capable of emitting, upon beam excitation, red, green or blue light. The photoscreening steps are repeated for each of the remaining colors to deposit triads of phosphor deposits on the target area in coordinate relationship with each aperture of the mask.
In the faceplate screening process, the phosphors for each color are typically embodied in a process screening fluid commonly referred to as a "slurry." The slurry is typically applied to the faceplate by a process known as "radial flow suffusion." The screening fluid is poured onto the faceplate while the faceplate is rotating. As the faceplate turns, the fluid spreads to the edges of the panel and excess fluid is cast off by centrifugal force. If there is any impediment to the free flow of the slurry during the screening process, the radially out-rushing slurry will "wash back," resulting in wave patterns in the coating which will become fixed following the drying of the slurry as by air and applied heat. The effect of this non-uniformity in phosphor density can become cumulative as the faceplate is successively screened. The deleterious effects of the wave patterns are three-fold. First, the thickened coatings are visible to the viewer as dark areas on the screen; second, cross-contamination of the colors can occur; and third, underexposure in the thickened areas during the photoprinting process results in non-adherence of the phosphor and consequent phosphor wash-off and flake-off.
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube having a thin foil mask sealed in tension directly to the bulb. Included in this disclosure is a description of the sealing of a foil mask between the junction of the skirt of the faceplate and the funnel. The mask is shown as having two or more alignment holes near the corners of the mask which mate with alignment nipples in the faceplate. The nipples pass through the alignment holes to fit into recesses in the funnel. In another Moore embodiment, the front panel is shown as having a continuous ledge around the inner surface of the faceplate. The top surface of the ledge is spaced a Q-distance away from the faceplate for receiving a foil mask such that the mask is sealed within the tube envelope. In yet another embodiment, there are two ledges located at the sides of the faceplate parallel with the vertical axis of the faceplate for receiving a shadow mask. Also shown is an embodiment in which the faceplate is skirtless and essentially flat.
Other prior art: Lerner--U.S. Pat. No. 4,087,717; Dougherty--U.S. Pat. No. 4,045,701; Palac--U.S. Pat. No. 4,100,451; Law--U.S. Pat. No. 2,625,734; Steinberg et al--U.S. Pat. No. 3,727,087; Schwartz--U.S. Pat. No. 4,069,567; Oess--U.S. Pat. No. 3,284,655; Hackett--U.S. Pat. No. 3,303,536; Vincent--U.S Pat. No. 2,905,845; Fischer-Colbrie--U.S. Pat. No. 2,842,696; a journal article; "The CBS Colortron: A color picture tube of advanced design." Fyler et al. Proc. of the IRE, Jan. 1954. Dec. class R583.6; and a digest article: "A High-Brightness Shadow-Mask Color CRT for Cockpit Displays." Robinder et al. Society for Information Display, 1983.